148 THREE-DIMENSIONAL PHOTOGRAPHYcan and will see. Therefore the formula can only give values
which apply to those individuals whose vision is accurately in
accord with the laboratory standards, perhaps one person in a
thousand, We have another formula which proves conclusively
that normal base stereo cannot be used at a distance nearer than-
well ten, eight, five feet, take your choice! One marvels indeed
at the power of mathematics-or should one say the dexterity of
the mathematician? There is a good, sound formula to prove just
about anything that anyone wishes to prove. Again the trouble
with the formula is that it is meaningless because it does not even
approximately conform to demonstrable fact.
What you can do with stereo depends upon your stereo sensi-
tivity, and with all due regard to the theorists, stereo sensitivity
involves more than stereopsis, at least more than the term stereop-
sis is assumed to include.
For example, there are two objects, both at a considerable dis-
tance; one is slightly nearer than the other. According to the
formula, the distance separating those two objects must be at least
such-and-such to enable you to see that one is nearer than the
other. When you actually look at such objects you (as a rule) can
easily see the separation and by careful observation you gain some
idea of the amount of distance between them. Here stereopsis is
superior to the formula. If they are moved closer together by a
certain amount, conditions change, When you first look at them
you can tell which is nearer, but if you study them you lose assur-
ance and cannot tell which is nearer. Here the tenuous stereo
sense has operated but stereopsis is now undependable. This
subtle sense is difficult to identify but the writer feels sure that it
is only the normal operation of dynamic parallax. As long as the
eyes move from object to object, the stereo sense is most acute,
and as soon as the eyes become fixed, the acuity drops. At least
this is true for general conditions and there is no reason why it
should not also explain the discrepancy between the delicacy of
the stereo sense, and that of stereopsis as usually defined by meas-
urement.
One thing is undoubtedly true, the conventional stereo limita-
tions are certainly far more restricted than is warranted by aver-
age normal vision. But we must remember that these limitations
were worked out by men leading a largely sedentary life, by stu-